The present invention generally relates to turbine components, including the turbine disks and seals of a gas turbine engine. More particularly, this invention relates to turbine disks and seals susceptible to oxidation and hot corrosion, and metallic environmental coatings that are adherent and compatible with disk and seal alloys and capable of providing protection from oxidation and hot corrosion.
The turbine section of a gas turbine engine contains a rotor shaft and one or more turbine stages, each having a turbine disk (or rotor) mounted or otherwise carried by the shaft and turbine blades mounted to and radially extending from the periphery of the disk. Adjacent stages of the turbine are separated by a non-rotating nozzle assembly with vanes that direct the flow of combustion gases through the turbine blades. Seals elements reduce leakage between the rotating and non-rotating (static) components of the turbine section, and channel cooling air flow to the turbine blades and vanes.
Turbine components are formed of superalloy materials in order to achieve acceptable mechanical properties at the elevated temperatures within the turbine section of a gas turbine engine. In particular, turbine airfoil components such as blades and vanes are often formed of equiaxed, directionally solidified (DS), or single crystal (SX) superalloys, while turbine disks and seal elements are typically formed of polycrystalline superalloys that undergo carefully controlled forging, heat treatments, and surface treatments such as peening to achieve desirable grain structures and mechanical properties. Though significant advances in high temperature capabilities of superalloys have been achieved, turbine components located in the hot gas flow path, such as the blades and vanes, are susceptible to damage by oxidation and hot corrosion attack, and are therefore typically protected by an environmental coating and optionally a thermal barrier coating (TBC), in which case the environmental coating is termed a bond coat that in combination with the TBC forms what may be termed a TBC system. Environmental coatings and TBC bond coats widely used on turbine blades and vanes include diffusion aluminide coatings and alloys such as MCrAlX overlay coatings (where M is iron, cobalt and/or nickel, and X is one or more of yttrium, rare earth elements, and reactive elements). The aluminum contents of diffusion aluminide and MCrAlX coatings are sufficient so that a stable and environmentally protective alumina (Al2O3) scale forms on their surfaces at the operating temperatures of turbine blades and vanes.
As operating temperatures of gas turbine engines continue to increase, the turbine disks and seal elements are also subjected to higher temperatures. As a result, corrosion of the disks/shafts and seal elements has become of concern. Corrosion of turbine disks has been attributed to deposition of solid particles containing metal sulfates or other metal sulfur oxides plus reducing agents, the reaction of the deposited particles with the disk alloy at high temperatures to form reduced metal sulfides covered by air-impermeable fused solid particles, and other corrosive agents including alkaline sulfates, sulfites, chlorides, carbonates, oxides and other corrodant salt deposits. Various corrosion barrier coatings have been investigated to prevent the corrosion of turbine disks from this type of attack. One such approach using layered paints has been hampered by the susceptibility of such paints to spallation during engine operation, believed to be caused by a significant CTE (coefficient of thermal expansion) mismatch between the layered paint and the alloy it protects, which results in high interfacial strains during thermal transient engine conditions. Adhesion of layered paints is likely limited in part by the reliance on mechanical adhesion between the paint and alloy, which can be improved to some extent by grit blasting the surface to be coated prior to depositing the paint. However, spallation remains an impediment to the use of layered paints. Other corrosion barrier coatings have been considered, including aluminides, chromides, and oxides deposited by, for example, metallo organic chemical vapor deposition (MO-CVD), pack silicides, ion implanted aluminum, metal nitrides, and metal carbides. Particular example of these approaches are disclosed in commonly-assigned U.S. Pat. Nos. 6,532,657, 6,921,251, 6,926,928, 6,933,012, and 6,964,791, and commonly-assigned U.S. Patent Application Publication Nos. 2005/0031794 and 2005/0255329.
In addition to corrosion, fatigue testing at elevated temperatures has shown that current disk alloys are also susceptibility to grain boundary oxidation if subjected to higher operating temperatures over extended periods of time. Therefore, in addition to protection from corrosion, higher turbine operating temperatures are necessitating the protection of turbine disks and seals from oxidation. Corrosion barrier coatings are not necessarily effective as oxidation barriers or inhibitors, particularly for extended exposures at high temperatures. Though the MO-CVD aluminide and chromide coatings and metallic carbide and nitride coatings noted above are also potentially capable of serving as barriers to oxidation, these corrosion barrier coatings are believed to have limitations that may render them unsatisfactory for use as protective coatings on turbine disks and seals, such as limited adhesion, CTE mismatch, low volume processing, and chemical interactions with the types of alloys often used to form turbine disks and seals. More particularly, though aluminide coatings exhibit excellent adhesion and corrosion resistance, they can negatively impact the fatigue life of a disk. Chromide coatings also exhibit great adhesion and corrosion resistance, as well as ductility (if the undesirable alpha-chromium phase does not form). However, high processing temperatures required to form chromide coatings make their use difficult on forged parts. Nitride and carbide coatings are generally subject to the same limitations noted above for aluminide and chromide coatings. Finally, oxide coatings (including those applied by MO-CVD) are excellent corrosion barriers and are not detrimental to fatigue properties, but their thermal expansion mismatch with superalloys limits their adhesion.
As such, there is a need for a protective coating material that is suitable for use on turbine disks and seals and resistant to oxidation and corrosion. Such a coating material must also be spall resistant and have an acceptable CTE match and limited mechanical property interaction with disk and seal alloys over extended time at high operating temperatures. In addition, such a coating material would ideally be compatible with the typical processing required for polycrystalline superalloys from which turbine disks and seals are formed.